1. Field of the Invention
The present invention relates to an image communication system using a network bus-line.
2. Description of the Prior Art
In general, factories or the like have a system for monitoring images from a plurality of predetermined places inside or outside the factories. The monitoring system has many cameras in the predetermined places to be monitored. Image data obtained by the monitoring cameras are collected in a central control device which has display monitors to display the image data in order to indicate whether abnormal or unusual events have occurred. Each camera is connected to the central control device through an individual line. The images from the predetermined places to be monitored are displayed on a display monitor selected by a selection switch.
FIG. 1 shows a block diagram of a prior art monitoring system including several monitoring cameras. This monitoring system comprises a central control device 20 and monitoring cameras C1, C2, C3, C4 and C5. The cameras C1, C2, C3, C4 and C5 are connected to the central control device 20 through their respective cable L1, L2, L3, L4 and L5. The central control device 20 has three monitoring televisions 25a, 25b and 25c, and selection switches 22a to 22e for the cameras C1 to C5. Image data obtained by the cameras C1 to C5 are sent to the central control device 20 through the cable lines L1 to L5.
In the central control device 20, the image data are selectively displayed on the monitoring televisions 25a, 25b and 25c. The following operation is carried out to display an image from the camera C4 and an image from the camera C5, when images from cameras C1, C2 and C3 are displayed on the monitoring televisions 25a, 25b and 25c, respectively.
At first, the selection switches 22a, 22b and 22c which correspond to the cameras C1, C2 and C3, are turned off. As a result of this operation, televisions 25a, 25b and 25c are disconnected from cameras C1, C2 and C3. Therefore, televisions 25a, 25b and 25c do not display any image. Next, the selection switches 22d and 22e which correspond to cameras C4 and C5, are turned on. As a result of this operation, television 25a is connected to camera C4 through cable L4, and television 25b is connected to camera C5 through cable L5. Therefore, television 25a displays the image from camera C4 and television 25b displays the image from camera C5. An image from any desired camera can be displayed by selecting the corresponding selection switch 22a to 22e.
However, the above mentioned monitoring system has the following problems.
To select an image from a camera, a corresponding switch is required for each camera. Therefore, it is difficult to increase the number of cameras beyond the number of selection switches.
As shown in FIG. 7, each camera C1, C2, C3 and C4 is connected to the central control device 20 through a respective cable L1, L2, L3 or L4. Accordingly, cable wiring becomes complex because a large number of cables are connected to the central control device 20. Also, it is difficult to change the location of camera C1, C2, C3 or C4 or to add an additional camera. For example, when a camera C5 is added in FIG. 7, cable L5 must be added between the camera C5 and the central control device 20. This problem is exacerbated when a large number of cameras are provided or cameras are located distant from the central control device 20.